The Persian idiom like a cow with a white forehead is used when one wants to characterize a person who is very well known in a crowd or even in a society.
Although it has a negative meaning, it does not mean notorious or ill-famed. Also, this definition that a person with a white forehead is someone who is very famous is not exact but not necessarily wrong. We use this idiom when we want to say that the person has done some works or has made some mistakes that have made him or her known. He or she has betrayed himself or herself.
This word is often used in a political context, when you want to say that this person is well-known to police and other security organizations.
Example: We can not rely on him. He is like a cow with a white forehead and may be arrested.
We can not rely on him. He is __________ [like a cow with a white forehead] and may be arrested.